1. Field of Invention
This invention relates generally to tethered hovering platforms (THP), and more specifically to tethered hovering platforms utilizing composite material, lightweight motors and lightweight high strength tethers.
2. Background Art
Inventors attempting to perfect early helicopter designs used tethers to add stability and safety during testing and development. The idea of using tethers was re-visited by the Germans and the French in World War II. The German Navy successfully deployed the Fa 330, an un-powered helicopter towed behind submarines. In the decade after WWII designs of extremely large, unmanned, single and multiple tethers hovering platforms such as the German KIEBITZ and the French Nord 510 were explored and prototypes were tested. These ideas were abandoned after only a few years of testing and development.
Hovering vehicles of today use onboard active stabilization systems for control. These systems after years of development have enabled vehicles such as the Sikorsky Cypher and Canadair C1-227 Sentinel to hover and fly in a stable manner and execute desired mission profiles. However, the stability of these craft comes at a very high cost, and the acquisition and maintenance costs of these craft are considerable. Intricate electronics of a complicated hovering vehicle include multiple piezoelectric gyroscopes, servos for actuation of control surfaces and motors, and occasionally onboard computers for autonomous flight. Two good examples of this type of vehicle are the Sikorsky Cypher and the Honeywell MAV. While these examples are very innovative, none have been commercially used for the missions that they were designed for. The problems with these concepts are that they are too expensive, difficult to maintain, have complicated deployment and recovery systems, and they are challenging to control.
Tethered observation balloons were first used for reconnaissance in the U.S. Civil War by both the Union and Confederate armies. Thaddeus Lowe helped develop the initial concept and helped establish the first balloon core, which he commanded. The first tethered observation balloon saw service on Sep. 24, 1861. On this date Lowe himself telegraphed intelligence about the Confederate troops at Falls Churce Va., over 3 miles from where his balloon was located. Un-powered Observation balloons remained in service with the both Allied and German ground and naval forces through WWII. These balloons became easy targets for aircraft and their usefulness was severely limited.
Tethered Rotorplatforms and Their Mission Potential by Wener Goller 1980 is an early publication on the topic of rotor platforms. The paper covers several topics of great importance to the topic at hand. The first is the definition and mission outline for these vehicles at the time. It states, “Tethered rotorplatforms are unmanned helicopters connected to the ground station by means of a tethering cable.” It goes on to define the basic advantage and mission of these types of crafts. “The main objective of tethered rotorplatforms is to elevate sensors or transmitters to an operation height at which the maximum range of these systems can be utilized without considerable restriction by terrain roughness and vegetation.” “The first tethered rotor platform, that was successfully flight tested up to a height of 45 m above the ground was constructed during World War I by Stefan Petroczy and Theodore von Karman.”
“It was tethered by three cables fixed at the end of three cantilevers of the fuselage and driven by three aeroplane motors of 120 hp each.” Petroczy and Karman's vehicle used a pilot/observer since no electronic means of performing the flight and observation tasks was available.
A similar 3 tether system was developed during World War II, by the German AEG company. Utilizing a 200 kw (268 HP) electric motor the craft could reach a maximum altitude of 750 m (2,460 ft) AGL. Their design was intended to carry one observer or a set of radio antennas. Development was stopped before the unit ever became operational.
During the post WWII years the Hiller Aircraft Company under contract with the office of naval research, developed the Hiller VZ-1 flying platform. Tethered flight experiments starting in 1953 eventually led to successful free flight in 1955. The craft used counter rotating ducted fan, upon which the pilot stood. The craft was controlled by the pilot shifting his weight in the direction he wanted the platform to move. Variants of several sizes were produced for analysis by the US Army, which after tested concluded the craft was impractical. The Hiller VZ-1 was limited to very slow speeds and calm conditions, and would not operate very far out of ground effect.
The first notable vehicle to undergo long term development is the German build Kiebitz. After a successful predecessor program in 1972, the German military awarded Dornier a contract to build and test the Kiebitz. The system had two main components, the ground station/base vehicle and the hovering platform; these two units were attached by the large single main tether.
Even at this point in history the rotorcraft used an onboard control system to give it defacto flight stability. Some of the exhaust gas was routed through a yaw control nozzle, and was used in conjunction with an onboard analog electric autopilot unit. The craft could operate in three control modes, attitude, position, and drift control.
The main cable attaching the flight vehicle to the ground station was quite complex since it carried jet fuel up to the turbine engine, as well as many electrical cables for control signals and sensor returns. This vehicle was designed to carry comparatively large sensor arrays into the air to detect aircraft or large ground vehicles approaching a position in central Europe. Alternatively it had options to operate as a mobile relay station for radio signals on the battle field. Since there are few obstacles on the open sea, an investigation of using the system aboard ships was made. The height advantage given by the vehicle would allow detection of ships at extreme range, and could even give over the horizon capabilities.
This vehicle was considered small and portable at the time, with a weight of several hundred pounds and a vehicle diameter of nearly 5 feet. By today's standards, the craft was quite large and very heavy. The materials used to construct the vehicle, as well as the propulsion system and payloads were orders of magnitudes higher in weight than modern systems.
Some have made mention of a small electric powered platform tethered by one tether that uses active control flaps. Similar ideas can be seen in some vehicles that are being developed today. Still as can be seen by the dimensions the craft is still quite large with a rotor diameter of 2.2 m. Some desired to see tethered rotorplatforms thrive and become useful for the perceived missions, but the opposite has seemed to happen. In fact they disappeared and were replaced with complex high dollar free-flight solutions.
The French developed an elegantly designed shrouded THP, called the Nord 510. It was powered by a turboprop engine, with a blade that was 5.9 ft in diameter. The platform was able to fly and hover above the ground vehicle up to a height of 1,000 ft. The vehicle had an empty weight of 990 lb, not including the weight of fuel or the tethers at altitude. The vehicle has an approximate diameter of 6 ft and a height of 8 ft, and is not much smaller than the DO32k. The ground vehicle and attached winch systems are approximately 12 ft tall and 25 ft long.
The “Rahfan” was a ducted fan free flying vehicle that used control tabs downstream of the propeller. The designers directly commented about the instability of the aircraft, and say that the design should be tailored to “off design conditions,” implying mainly to windy weather conditions. The comments allude to the conclusion that the craft is not stable in a wind gust and suffers the same instabilities as other ducted fan UAV's of similar configuration.
In the paper Development of a Shrouded-Fan UAV for Environmental Monitoring the authors say that it does not really fly at all. The Department of Aerospace Engineering at the Polytechnic of Turin in Italy developed a new ducted fan vertical takeoff and landing (VTOL UAV), where the whole vehicle is enclosed in one large shroud. The vehicle uses a large gas engine for power. Work on the project began in 1997 and the paper was published in 2004. The performance specifications are noted as such.
“The UAV, has a maximum takeoff weight of 1,000 N (224 lb), and the shroud diameter and weight are 1.9 m (6.2 ft) and 110 N (25 lb), respectively. Expected maximum performance is as follows: Scientific payload weight 100 N (24.7 lb), altitude 2,000 m (6561 ft), speed 30 m/s (67 mph), range 100 km (62 ml), endurance 2 hours.”
“Unfortunately, this system can be used only for tests of relatively small duration, where the vehicle remains in a nearly horizontal attitude, that is, hovering and low speed translation. If the fuel consumption of the engines is slightly different, an unbalanced weight distribution can result on the long run. The same unbalance can also be induced by the failure of one of the engines. Finally, fuel must be provided to all the three engines in any flight condition, and this may be difficult when the tanks are only partially filled and the vehicle has a pitch attitude of more than 20 deg, as in forward flight at high speed.”
The designers at the Polytechnic of Turin vehicle have not addressed any pitch back instability due to the large shroud, or issues with center of gravity placement on the vertical axis. There is also little shown on the control mechanisms, whether they are control vanes or thrust vectoring or another system as seen on current free flight vehicles.
The design is very similar to the Sikorsky Cipher. The first proof of concept Cypher was flown in 1988, with the first free flight prototype taking flight in 1993. This vehicle went through extensive testing and development throughout the 1990s and led to development of the Cypher II The Sikhorsky Cipher never reached production stage, but the program may still be active.
The Canadair CL-227 Sentinel was first developed in 1978. Development of the Sentinel progressed through three separate phases over the span of over ten years. The CL-227 is a remotely piloted vehicle with a turboshaft engine that drives counter rotating rotors. The CL-227 can operate in tethered or free flight modes. The phase three variant is roughly 5 feet tall, weighs 419 lb (190 kg), and has a payload capacity of 99 lb (45 kg). It can reach a maximum level speed of 81 mph (130 km/h) and has a typical mission endurance of 3 to 4 hours.
The Sentinel was upgraded to the CL-327 Guardian. The Guardian offers approximately 6 hours of endurance, a maximum altitude of 18,000 ft, and an increased payload capacity of 220 lb. The Guardian is equipped with state of the art radar, optical sensors, and communications equipment, and is in production by the Bombardier Company.
The paper Improving Control System Effectiveness for Ducted Fan VTOL UAVS Operating in Crosswinds optimizes what has become the standard approach in the development of hovering platform type systems. It seems from all the vehicles being developed that they want a fully autonomous self controlled and stabilizing craft. For some missions this is indeed necessary; however for relatively close-in surveillance other options exist outside this paradigm. The instability problems of ducted fan VTOL UAVs, problems still haunt even the most successful modern vehicles. “There are two significant, inherent issues associated with ducted fan control in crosswinds; 1) lateral momentum drag and 2) a duct stabilizing torque which resist tipping into the wind.” The authors worked on many projects including a DARPA funded project in conjunction with Honeywell called the “Kestrel” Organic Air Vehicle.
The instabilities and control of these types of free flight craft are so complicated that in addition to the CFD models a 6 degree of freedom test model was created and wind tunnel tested. Some really substantial work was conducted and involved a lot of time and money thrown at the same problem that has existed since the first ducted fan VTOL UAV. The following four pictures show how in a strong crosswind the conventional use of control vanes is problematic as the vanes loose effectiveness unevenly due to thrust cone bending. These images also demonstrate how these control vanes should be placed very close to the rotor plane to not lose effectiveness. However, placement in such a position decreases their moment arm and thus reduces significantly their control effectiveness.
These types of instabilities, and control issues caused by insufficient control power posed large challenges to the designers and engineers working on a piloted ducted fan vehicle in the 1950's.
Aero-Design & Development out of Israel has released a vehicle extremely similar to the Hiller VZ-1 concept, now named the AD & D Hummingbird. They performed tethered flight tests starting in August of 1997, and conducted free flight testing in October of 1998. Sale of the craft in kit form was scheduled to take place in 2000, but as of May 2008 this craft is still not for sale for unknown reasons.
The state of the art free flight vehicle being deployed to Iraq and Afghanistan is the Honeywell Micro Air Vehicle (MAV). This vehicle is the product of a long term DARPA development contract whose earlier vehicles included the Kestrel and Organic Air Vehicle (OAV). “The MAV is a scout reconnaissance type vehicle. It allows people to see over a hill or around a building.” The vehicle weighs 12.5 lbs, is capable of operation in 20-knot winds, and has 40 minutes of endurance at 5,500 ft. The motors, ducted fan central body and other components can be recognized.
While this free flight vehicle is capable of flying at 50 kts airspeed and climbing at 25 feet/second, the mission profile for the vehicle is described as “hover and stare”. The hover and stare mission profile consists of vertical takeoff and then hovering above a place of interest providing elevated reconnaissance images. The vehicle is equipped with forward and downward looking visual and infrared camera system. The 2nd Battalion, 5th Infantry Regiment has been undergone several months of training with the vehicle in the Military Operations in Urban Terrain training site.